Investigation Expérimental et Numérique sur l’Endommagement des Grains d’un Sable soumis à des Fortes Contraintes : Effet de Fluage

Abstract

Granular materials, originating from either natural sources (such as sand and gravel) or artificial ones (such as ballast and riprap), are widely used in various fields, including civil engineering, the pharmaceutical industry, and mining. When subjected to external loading, their macroscopic response reflects changes occurring at the microscopic scale, particularly at the level of individual particles. Phenomena such as landslides, which result from particle scale breakage, clearly illustrate the significant influence of these microscopic changes on the overall behavior of granular materials. This observation underscores the growing interest in investigating particle breakage mechanisms, a key area of research in geomechanics. The objective of this doctoral research is to investigate the impact of grain breakage on the evolution of microstructural properties, such as porosity, specific surface area, and pore size distribution. This study also focuses on the evolution of micromechanical contact properties, particularly coordination number and interparticle contact surface area. In this context, high-pressure oedometer tests were conducted on a carbonate sand subjected to various stress levels. These tests were followed by grain size distribution analyses and permeability measurements at each applied stress level. In this study, X-ray tomography was utilized to acquire three-dimensional (3D) images of carbonate sand samples, a granular material, subjected to high-pressure oedometer testing. Four X-ray tomography acquisitions were carried out on the same sample at increasing stress levels of 0, 2.5, 5, and 10 MPa. The results of the analysis of the 3D images obtained by X-ray tomography reveal a decrease in porosity and average pore size, as well as an increase in specific surface area. Furthermore, the increase in load leads to an increase in the contact surface area due to particle breakage, reaching an increase of 14.18% under a stress of 10 MPa. The coordination number is influenced by the particle fracture mode and the grain size and brittleness: larger particles have a high coordination number, unlike smaller fragments. For intact grains, this number increases with the applied load. This evolution, combined with an increased contact surface area, contributes to strengthening the resistance of the sample to shear and compressive stresses. The results obtained allowed to establish an empirical relationship between the void ratio and the specific surface area (SSA). This relationship can be used to relate microstructural properties (specific surface area) to macroscopic characteristics (void ratio),